question_answer

Water of volume 2 L in a closed container is heated with a coil of 1 kW. While water is heated, the container loses energy at a rate of 160 J/s. In how much time will the temperature of water rise from 27°C to 77°C? (Specific heat of water is 4.2 kJ/kg and that of the container is negligible).                                     [JEE ONLINE 09-04-2014]

A)                         8 min 20 s        

B)       6 min 2 s

C)  7 min  

D)       14 min

Correct Answer: A

Solution :

[a] From question,

In 1 sec heat gained by water

= 1 KW – 160 J/s

= 1000 J/s – 160 J/s

= 840 J/s

Total heat required to raise the temperature of water (volume 2L) from 27°c to 77°c

[Q mass = density  volume]

And,

or